A natural-gas leak near San Francisco's Moscone Center on Sunday afternoon tied up traffic in the South of Market neighborhood and forced authorities to evacuate hundreds of people from nearby homes and businesses.

The leak was reported shortly after noon near the intersection of Fourth and Folsom streets and was apparently caused by construction workers' accidentally cutting into a Pacific Gas and Electric Co. gas line, said San Francisco police Officer Albie Esparza.

While PG&E dispatched workers to assess the leak and shut down the line, officials with the San Francisco police and fire departments closed off nearby streets and started evacuating people within a one-block radius of the leak. Several blocks of Folsom and Harrison streets were closed off.

No fire or explosion occurred, however.

"Initially, we thought the leak was a lot worse than it is," Esparza said, as local residents and employees of shuttered businesses milled around the intersection of Folsom and Fifth streets.

"I know it's an inconvenience, but we're doing it for a reason." Esparza said. "You never know with gas."

PG&E workers had to dig down below the street to reach the leaking 6-inch gas line and shut it off, said company spokesman J.D. Guidi. The flow of gas was finally shut off at 2:48 p.m. he said.

Esparza did not know how many people had been evacuated, saying it was at least 300 or 400. He also was not certain if people in the nearby Metreon - frequently packed with moviegoers on weekends - had been evacuated or told to shelter in place.

John Garland, with the local stagehands union, had been working Sunday at the Moscone Center, which is hosting a convention of endocrinologists. Around 12:30 or 12:45, the center's emergency warning system went off, he said.

Workers had been told that roughly 7,900 people were attending the convention, he said.

"We're very fortunate," Garland said, referring to the lack of explosion or fire. "With all those people in there, it could have been a real disaster."

PG&E's response to natural gas leaks has come under sharp scrutiny ever since a much larger, 30-inch gas transmission line in San Bruno exploded on Sept. 9, 2010, killing eight people and destroying 38 homes. Sunday's leak was reported at 12:09 p.m., and the first PG&E employees arrived on the scene at 12:32, Guidi said.

Construction crews at that intersection have been working on building a station for the new Central Subway, which will run up Fourth Street from the Caltrain Station. Neither Esparza nor Guidi knew whether the construction workers who accidentally hit the gas distribution line Sunday were part of that project.